The isometric twitch of right ventricular papillary muscles from rabbits with right ventricular hypertrophy secondary to pulmonary artery banding manifests a longer time from the onset of the twitch to its peak and a decreased maximum rate of tension development when compared with normal. Series elasticity is unchanged. Consequently hypertrophic growth may result in a depressed rate, but prolonged duration of internal activator Ca2 ion release. The duration of isometric relaxation is unchanged so that the time course of internal calcium sequestration is probably not altered. A servo-controlled system will be used to length-clamp sarcomeres. Instantaneous sarcomere length will be determined by laser diffraction patterns that will be transduced from optical to electrical signals and used as part of a negative feedback servo loop. Constant length sarcomere tension development will be studied as an index of internal Ca2 ion release in very thin right ventricular papillary muscles from young rabbits with pulmonary artery constriction as compared with sham-operated and normal controls. Temperature change, pharmacologic interventions, and/or rate treppe will be used to evaluate excitation-contraction coupling as manifested in instantaneous sarcomere force levels during the onset of the twitch in the two types of heart muscle. Mechanical measurements will be obtained within hours to days postoperatively in banded and sham-operated rabbits to evaluate whether changes in isometric force development precede depression of the force-velocity relationship. The proposition is that the initial mechanical defect in cardiac hypertrophy resides primarily in the excitation-contraction coupling system. Altered excitation-contraction coupling in hypertrophy is manifested as time related changes in isometric force development that will be investigated at the level of the sarcomere. Isotonic sarcomere length change also will be studied to extend my previous observations of the force-velocity relationship in normal heart muscle and in established myocardial hypertrophy.